Design & Operating Notes:
Normally, this extremely high input impedance of greater than 1014Ω
would not be a problem as the source impedance would limit the node
impedance. However, for applications where source impedance is
very high, it may be necessary to limit noise and hum pickup through
proper shielding.
1. The ALD1731A/ALD1731 CMOS operational amplifier uses a 3 gain
stage architecture and an improved frequency compensation
scheme to achieve large voltage gain, high output driving capability,
and better frequency stability. In a conventional CMOS operational
amplifier design, compensation is achieved with a pole splitting
capacitor together with a nulling resistor. This method is, however,
very bias dependent and thus cannot accommodate the large range
of supply voltage operation as is required from a stand alone CMOS
operationalamplifier. TheALD1731A/ALD1731isinternallycompen-
sated for unity gain stability using a novel scheme that does not use
a nulling resistor. This scheme produces a clean single pole roll off
in the gain characteristics while providing for more than 70 degrees
of phase margin at the unity gain frequency.
4. The output stage consists of class AB complementary output drivers,
capable of driving a low resistance load. The output voltage swing is
limited by the drain to source on-resistance of the output transistors
as determined by the bias circuitry, and the value of the load resistor.
When connected in the voltage follower configuration, the oscillation
resistant feature, combined with the rail to rail input and output
feature, makes an effective analog signal buffer for medium to high
source impedance sensors, transducers, and other circuit networks.
2. The ALD1731A/ALD1731 has complementary p-channel and n-
channel input differential stages connected in parallel to accomplish
rail to rail common mode input voltage ranges. This means that with
the ranges of common mode input voltage close to the power
supplies, one of the two differential stages is switched off internally.
To maintain compatibility with other operational amplifiers, this
switching point has been selected to be about 1.5V below the positive
supply voltage. Since offset voltage trimming on the ALD1731A/
ALD1731 is made when the input voltage is symmetrical to the supply
voltages, this internal switching does not affect a large variety of
applications such as an inverting amplifier or non-inverting amplifier
with a gain larger than 2.5 (5V operation), where the common mode
voltage does not make excursions below this switching point. The
user should, however, be aware that this switching does take place if
the operational amplifier is connected as a unity gain buffer and
should make provisions in the design to allow for input offset voltage
variations.
5. The ALD1731A/ALD1731 operational amplifier has been designed to
provide full static discharge protection. Internally, the design has
beencarefullyimplementedtominimizelatchup. However,caremust
be exercised when handling the device to avoid strong static fields
that may degrade a diode junction, causing increased input leakage
currents. In using the operational amplifier, the user is advised to
power upthecircuit before, or simultaneouslywith, any input voltages
applied and to limit input voltages not to exceed 0.3V of the power
supply voltage levels.
6. The ALD1731A/ALD1731, with its micropower operation, offers
numerous benefits in reduced power supply requirements, less
noise coupling and current spikes, less thermally induced drift,
better overall reliability due to lower self heating, and lower input
bias current. It requires practically no warm up time as the chip
junction heats up to only 0.1°C above ambient temperature under
most operating conditions.
3. The input bias and offset currents are essentially input protection
diodereversebiasleakagecurrents, andaretypically0.01pA atroom
temperature. This low input bias current assures that the analog
signal from the source will not be distorted by input bias currents.
7. The ALD1731A/ALD1731 has an internal design architecture that
provides robust high temperature operation. Contact factory for
custom screening versions.
TYPICAL PERFORMANCE CHARACTERISTICS
SUPPLY CURRENT AS A FUNCTION
OF SUPPLY VOLTAGE
COMMON MODE INPUT VOLTAGE RANGE
AS A FUNCTION OF SUPPLY VOLTAGE
±6
500
400
INPUTS GROUNDED
OUTPUT UNLOADED
+25°C
±5
T
= 25°C
A
-25°C
±4
300
T
A
= -55°C
±3
±2
±1
200
100
+125°C
±5
+70°C
0
0
0
±1
±2
±3
±4
±5
±6
0
±1
±2
±3
±4
±6
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
INPUT BIAS CURRENT AS A FUNCTION
OF AMBIENT TEMPERATURE
OPEN LOOP VOLTAGE GAIN AS A
FUNCTION OF LOAD RESISTANCE
1000
100
1000
100
10
V
= ±2.5V
S
10
1.0
V
T
= ±2.5V
= 25°C
S
A
0.1
0.01
1
10K
-50 -25
0
+25
+50
+75
+100 +125
100K
1M
10M
AMBIENT TEMPERATURE (°C)
LOAD RESISTANCE (Ω)
ALD1731A/ALD1731
Advanced Linear Devices
4 of 9
ALD [ ADVANCED LINEAR DEVICES ]
